Battery modules including a plurality of cells accommodated in a case to be capable of outputting a predetermined voltage and capacitance are widely used as power supplies of various devices, vehicles, etc. and household power supplies. Specifically, the technique of forming blocks by connecting general-purpose secondary batteries in parallel and/or in series to be capable of outputting a predetermined voltage and capacitance and being charged, and combining the battery blocks in many ways to be applicable to various applications is beginning to be used. In the block formation technique, the performance of the batteries accommodated in the battery blocks is enhanced to reduce the size and the weight of the battery blocks themselves. Thus, the block formation technique has various advantages such as improvement of workability in assembling battery modules, and improvement of flexibility in mounting the battery modules in areas of limited space, such as a vehicle.
It is required for the cells included in the battery modules as described above to have substantially the same capacitance (dischargeable electric capacitance at a certain point in time). This is because if the cells exhibit variations in capacitance, certain cells are subjected to a high charge/discharge load compared to the other cells, so that the certain cells degrade earlier, which results in reduced life of the battery modules themselves.